Back around 2019, I had a little project at work where I had to clean up non-stop communication errors between our generator control panel and two sets of remote monitoring devices..
The original communication system for the breakers in the Burrard breaker room was Modbus over RS-485 using twisted pair. The original communication system for the power monitors in the south distribution room was also Modbus over RS-485 using twisted pair.
The system had two IP-to-Modbus gateways in the generator control panel.
One gateway communicated with the breakers through about 120 metres of cable, and the other gateway communicated with the meters through about 220 metres of cable.
As I said before, both networks were throwing up non-stop errors and alarms. Mainly checksum errors, but also plenty of timed out errors and outright communication failed errors.
There are two common types of cable that can be used with an RS-485 network.
Twisted pair with shield, or twisted pair with common and shield.
Twisted pair with shield is fine for networks where all of the devices on share either the same power supply or the same common ground reference.
Twisted pair with common is used where the devices do not share the same common power supply or the same common reference to ground. This is the kind of situation you can run ito with VFDs, or devices that are located a fair distance away from the main system and only share a common bond through the safety ground.
All three locations were powered from separate power sources. Yes, all three buildings share a common bonded ground, but that bonding is for electrical safety. It is not there to provide a clean common reference for RS-485 transceivers.
The shield for the RS-485 wiring was installed properly. It was earth grounded at one end only and isolated at the other. So, there shouldn’t have been any current flow on the shield due to differences in ground potential in the two buildings.
The power meter network used cable with a common wire, but that network was having just as many problems as the breaker network that did not have the common wire.
A cable tester did not find any faults with the either cable.
So that left me with a head-scratcher.
Do I pull out all of the RS-485 cable and replace it, or do I get fancy and put the lino on fibre and never have to worry about ground loops, common references, electrical noise, or other weirdness that could pop up later?
I went with full-duplex fibre.
I had our network contractor pull 130 metres of six-strand fibre, install a fibre termination box at both ends, and dress and terminate two of the strands.
Then I installed a Moxa IP-to-Fibre media converter at each end of the fibre.
The other end of the fibre, in the Burrard switchgear room, is where I had to get a little fancy.
I had to install an IP-to-Fibre media converter, a five port switch, and two MOXA MB 3180 IP-to-RS-485 Modbus gateways. The MOXA gateways were assigned the same IP addresses as the original gateways they replaced in the generator control panel.
The reason the two gateways work at the end of a single IP-to-Fibre connection is that the switches used inside the generator control panel are unmanaged and unrouted. When either of the two PLCs or HMIs wants to poll devices on the network, it sends the traffic for the intended IP address and only the device with the matching IP responds.
I had Gerryl whip up a box with a power supply and some terminal blocks, then mount it using Unistrut extensions. Then I jammed the networking equipment into the box.
The nice thing is that most of the devices in this box are parts I already carry in stock in the Power Engineer’s workshop. The MOXA MB3180s are gateways I have used in various other Modbus projects at work. The five-port Ethernet switches are something that I already use for other projects. And the Mean Well power supplies are also devices that I use elsewhere. The DIN rail blocks are something that I keep in stock. The only new devices that I had to order were the MOXA IP-to-fibre converters.
The breakers in this the Burrard switchgear room form one node and go into one MOXA MB3180. The power monitors in the other electrical room, located about another 100 metres away, form the other node and go into the other MOXA MB3180.
I re-used the original RS-485 wiring from the breakers the Burrard switchgear room to this box, and I reused the original RS-485 wiring from the power monitors to this box.
Sure, it probably could have looked much prettier, but I’m a Power Engineer running an occupied facility, Jim! I’m not a miracle worker.
My job was to stop the non-stop barrage of alarms that were being sent to the Shift Engineer’s pager.
The alarms stopped, and we get proper readings, so my job is done.
And in a couple of year, this will all be long gone, like it never even existed in the first place.
